1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more particularly to CMOS (complementary metal oxide semiconductor) devices having gate electrodes with a single work function.
2. Discussion of Related Art
During the past two decades, the physical dimensions of MOSFETs have been aggressively scaled for low-power, high-performance CMOS applications. In order to continue scaling future generations of CMOS, the use of metal gate electrode technology is important. For example, further gate insulator scaling will require the use of dielectric materials with a higher dielectric constant than silicon dioxide. Devices utilizing such gate insulator materials demonstrate vastly better performance when paired with metal gate electrodes rather than traditional poly-silicon gate electrodes.
Depending on the design of the transistors used in the CMOS process, the constraints placed on the metal gate material are somewhat different. For a planar, bulk or partially depleted, single-gate transistor, short-channel effects (SCE) are typically controlled through channel dopant engineering. Requirements on the transistor threshold voltages then dictate the gate work-function values must be close to the conduction and valence bands of silicon. For such devices, a “mid-gap” work function gate electrode that is located in the middle of the p and n channel work function range is inadequate. A mid-gap gate electrode typically results in a transistor having either a threshold voltage that is too high for high-performance applications, or a compromised SCE when the effective channel doping is reduced to lower the threshold voltage. For non-planar or multi-gate transistor designs, the device geometry better controls SCE and the channel may then be more lightly doped and potentially fully depleted at zero gate bias. For such devices, the threshold voltage can be determined primarily by the gate metal work function. However, even with the multi-gate transistor's improved SCE, it is typically necessary to have a gate electrode work function about 250 mV above mid-gap for an nMOS transistor and about 250 mV below mid-gap for a pMOS transistor. Therefore, a single mid-gap gate material is also incapable of achieving low threshold voltages for both pMOS (a MOSFET with a p-channel) and nMOS (a MOSFET with an n-channel) multi-gate transistors.
For these reasons, CMOS devices generally utilize two different gate electrodes, an nMOS electrode and a pMOS electrode, having two different work function values. For the traditional polysilicon gate electrode, the work function values are typically about 4.2 and 5.2 electron volts for the nMOS and pMOS electrodes respectively, and they are generally formed by doping the polysilicon material to be either n or p type. Attempts at changing the work function of metal gate materials to achieve similar threshold voltages is difficult as the metal work function must either be varied with an alloy mixture or two different metals utilized for n and p-channel devices.
One such conventional CMOS device 100 is shown in FIG. 1, where insulating substrate 102, having a carrier 101 and an insulator 103, has a pMOS transistor region 104 and an nMOS transistor region 105. The pMOS device in region 104 is comprised of a non-planar semiconductor body 106 having a source 116 and a drain 117, a gate insulator 112 and a gate electrode 113 made of a “p-metal” (a metal having a work function appropriate for a low pMOS transistor threshold voltage). The nMOS device in region 105 is comprised of a non-planar semiconductor body 107 having a source 116 and a drain 117, a gate insulator 112 and a gate electrode 114 made of an “n-metal” (a metal having a work function appropriate for a low nMOS transistor threshold voltage). While fabricating transistors having gate electrodes made of two different materials is prohibitively expensive, simpler approaches to dual-metal gate integration like work-function engineering of molybdenum, nickel and titanium through nitrogen implantation or silicidation suffer from problems such as poor reliability and insufficient work-function shift. However, as previously described, if a single mid-gap metal is used as the gate electrode for both the pMOS and nMOS transistors, the transistors have not had the low threshold voltage required for advanced CMOS.